1. Field of Invention
The present invention relates to a method for forming a silicon oxynitride layer. More particularly, the present invention relates to a method for forming a silicon oxynitride layer that has a higher etch-removal rate in subsequent etching process. This is accomplished through changing some parameters during the deposition of silicon nitride.
2. Description of Related Art
Silicon oxynitride (SiO.sub.x N.sub.y) is a dielectric material that has properties ranging between silicon dioxide (SiO.sub.2) and silicon nitride (Si.sub.3 N.sub.4). The internal stress of a silicon oxynitride layer is smaller in magnitude than a silicon nitride layer, while its resistance against moisture and impurities is better than a silicon dioxide layer. Therefore, a silicon oxynitride layer is commonly employed as a protective material. To avoid any serious damages to the metallic layer above the devices due to a high processing temperature during silicon oxynitride deposition, a processing temperature lower than 450.degree. C. is preferred. Hence, the silicon oxynitride layer is usually deposited using a plasma-enhanced chemical vapor deposition (PECVD) method with silane (SiH.sub.4), nitrous oxide (N.sub.2 O), and nitrogen (N.sub.2) as the reactive gases. The refractive index of silicon oxynitride is about 1.75, which is intermediate between silicon dioxide and silicon nitride. The refractive index is actually dependent upon the amount of oxygen and nitrogen inside the silicon oxynitride.
In photolithographic processing that uses deep ultra-violet (DUV) light source, depositing a thin layer of silicon oxynitride is a simple and economical way to form an anti-reflection layer (ARL). In general, the silicon oxynitride layer does not need to be removed. However, in performing a self-aligned silicide (Salicide) process, any coated silicon oxynitride layer must be completely removed.
Silicon oxynitride layer can be removed using hot phosphoric acid solution. However, prolonged exposure to phosphoric acid can damage the device's polysilicon layer and the substrate. Therefore, a faster etch-removal rate for the silicon oxynitride layer is very much preferred. The silicon oxynitride layer deposited using a conventional deposition method has a rather low etch-removal rate. Because of the slow etching rate, a few minutes of etching time is usually required to etch clean of any silicon oxynitride material. By the time the etching is finished, severe damages to the devices may result leading to a low product yield. On the other hand, if the etching time is shortened, residual silicon oxynitride will remain. This will adversely affect subsequent self-aligned silicide processing operation, which is equally detrimental to the device's reliability.
In light of the foregoing, there is a need in the art to improve the method of depositing silicon oxynitride to form silicon oxynitride layer.